A wavelength tunable laser is a laser capable of adjusting an emission wavelength. This wavelength tunable laser is used for a light source for optical communications, gas sensors, biosensors, and the like. The wavelength tunable laser to be used for optical communications, various sensors, and the like should have a wide wavelength tuning range, high output power, fast wavelength tuning rate and low cost.
Wavelength tuning methods for use in the wavelength tunable laser are a method using a vertical cavity surface-emitting laser (VCSEL) and a micro-electro-mechanical system (MEMS), a method using an external cavity laser with an external resonator, a method using a hot wire, a method using a distributed Bragg reflector (DBR) laser, and the like.
Among these methods, the method using the MEMS adjusts an output wavelength by changing a cavity length by using an MEMS technique. This method is advantageous in that a wavelength tuning range is wide and a wavelength is continuously tunable, but is disadvantageous in that a wavelength tuning rate is slow and stability is problematic upon long time use.
The method using the external resonator changes an output wavelength of a laser by mounting a wavelength selection mechanism of a prism or grating in the resonator including a laser and a mirror and moving the wavelength selection mechanism. Using this method, the wavelength tunable laser is advantageous in that an output power is large, an output light line width is narrow, a wavelength tunable range is wide, and a wavelength is continuously tunable, but is disadvantageous in that a wavelength tuning rate is slow and stability of a mechanical drive device is problematic.
The method using the hot wire changes an output wavelength by mounting the hot wire on a laser and adjusting a temperature of the laser using the hot wire. When a distributed feedback (DFB) laser is used in the method using the hot wire, a wavelength tuning coefficient according to a temperature is about 0.1 nm/C. When the DFB laser is used, a wavelength tuning width available in one DFB laser is limited. For this reason, there is a problem in that multiple DFB lasers are required when a large wavelength tuning width is required.
FIG. 1 is a view illustrating an embodiment of a conventional wavelength tunable laser diode using a hot wire. Referring to FIG. 1, a hot wire heater 112 and electrodes 110 and 111 of the hot wire heater 112 are deposited on a laser diode surface. That is, the hot wire is connected to a power supply and an emission wavelength is changed by heating a laser diode with the heat emitted from the hot wire. The hot-wire type DFB laser manufactured in this method is currently being commercialized. This method is advantageous in that a wavelength is continuously tunable and a structure is simple, and a production is easy, but is disadvantageous in that a wavelength tuning rate is slow.
On the other hand, the DBR laser is mainly configured with a gain region, a grating region and a phase region. Each region is driven using a separate current source, a carrier concentration varies with a current across each region, and an emission wavelength is changed by a variation of a refractive index based on the carrier concentration variation. This method may not continuously vary a wavelength. In order to obtain the specific wavelength light, a current corresponding to a desired wavelength should be provided by inspecting in advance an output wavelength based on a current across each region and creating its table. This method is disadvantageous in that it is complex to control a wavelength.